Influence of machining parameters on the subsurface damage of a high-strength silicon nitride, Si(3)N(4).

摘要

Grinding forces produced during creep feed grinding can have significant effects on a material's strength and Weibull modulus. Machining parameters, such as down feed, grit size, table speed and wheel surface speed, define the magnitude of the grinding forces. The grinding parameters and the properties of the workpiece determine what type of grinding process occurs, either ductile or brittle grinding. The ductile grinding process leads to larger amounts of localized subsurface plastic deformation. The magnitude of this subsurface deformation is directly related to the machining parameters, namely the grit size, the depth of cut, and the table speed. Compressive residual stresses are formed due to the localized plastic deformation, which can be beneficial to the overall strength of the workpiece. In order to balance the compressive stresses, tensile stresses are also formed which can be detrimental to the strength. The depth of this damage varies and needs to be correlated with respect to the machining parameters and grinding forces. Test specimens have been developed to reveal this damage so that it can be quantified. Scanning electron microscopy (SEM), transmission electron microscopy (TEM), and x-ray diffraction are utilized to determine the depth and type of damage.; The results show that the grit size was the dominate factor in altering the strength of the material. The machining-induced damage was not distinguishable from the rest of the fracture surface, making the determination of flaw size difficult. From fracture mechanics equations the flaw size was calculated and compared to the depth of plastic deformation. The plastic deformation was isolated to the upper grain layer of the material due to the nonaccomodation of plastic flow to the lower grains and also due to the low thermal conductivity of the material. The calculated flaw sizes were determined to lie at deeper depths than the plastic deformation, thus, the strength of the material was not enhanced by the plastically deformed layer.